1. Field of the Invention
The present invention relates generally to data processing systems and, in particular, to a processing platform that provides integrated general purpose and digital signal processing (DSP) capabilities for recovering and processing digital data utilizing an internal shared memory resource.
2. Discussion of the Prior Art
The basic function of any communications system is to transmit information over a communication channel from an information source to a destination as fast and as accurately as possible.
There are two general types of information sources. Analog sources, such as a telephone microphone, generate a continuous signal. Digital sources, such as a digital data processing system, generate a signal that consists of a sequence of pulses.
Communications channels that are designed to transmit analog signals (e.g., the telephone network) have characteristics which make it difficult for them to transmit digital signals. To permit the transmission of digital pulse streams over an analog channel, it is necessary to utilize the digital data pulses to modulate a carrier waveform that is compatible with the analog transmission channel.
The equipment that performs the required modulation is generally referred to as a "MODEM". The term "MODEM" is an acronym for MOdulator-DEModulator, since one piece of equipment typically includes the capability not only to modulate transmitted signals, but also to demodulate received signals to recover the digital data from the modulated analog carrier waveform.
While passing through the transmission channel, the modulated carrier waveform suffers from distortion introduced both by the system itself and by noise contamination. Thus, one of the tasks of the modem's demodulation function is to filter the signal received from the transmission channel to improve the signal-to-noise ratio. The demodulator also retrieves timing information from the received signal to provide sampling points for recovering the digital data. The demodulator may also condition the data in other ways to make it suitable for additional processing.
In a conventional modem, the signal filtering, sampling and conditioning tasks are performed by three functional units: analog-to-digital conversion circuitry ("analog front end") that converts the received modulated carrier waveform to a digitized replica, a digital signal processor (DSP) that recovers the digital data from the digitized replica, and a control function for controlling both the analog front end and the digital signal processor. The digital signal processor recovers the data by implementing a signal conditioning and data recovery algorithm that is specific to the type of data being received.
For example, the digital signal processor function in a facsimile (fax) machine modem implements a special purpose algorithm that can only be used for recovering digital fax data. In the case of a fax system, the data to be recovered is a digital bit map that corresponds to the transmitted hard copy image and which has been compressed to facilitate efficient transmission. The algorithm implemented by the digital signal processor function of the receiving fax machine's modem is a dedicated "fax" algorithm that has been designed specifically for accurately recovering the compressed bit map. It cannot recover digital data in a format other than a compressed bit map, e.g. voice mail data or data modem applications. A different digital signal processor implementing a different dedicated "voice mail" or "data modem" algorithm is needed for each of these other applications.
As shown in FIG. 1, a conventional fax machine architecture may be partitioned into two major functional blocks: (1) a special purpose fax modem block of the type described above for recovering a compressed bit map from a modulated carrier waveform and (2) a general purpose processor block for performing those tasks require to convert the compressed bit map to a corresponding hard copy image.
A well known example of a special purpose fax modem block is the Rockwell R96DFX MONOFAX.RTM. modem chip, the so-called "Rockwell Module". In the Rockwell Module, the incoming modulated carrier waveform received from an analog channel, i.e., a telephone line, is processed by an analog front end which generates a digitized replica of the analog signal; that is, the analog front end generates a digital reading of the input voltage level. A dedicated fax digital signal processor then performs the adaptive filtering, signal sampling, synchronization and carrier phase/frequency tracking required to reconstruct the compressed facsimile bit map from the digitized replica provided by the analog front end. The recovered bit map is then provided to the general purpose processor block which performs the additional processing functions required for printing the transmitted image. That is, the general purpose processor block controls and performs the data decompression, decoding, imaging and printing functions necessary to generate a hard copy reconstruction of the recovered bit map.
To transmit an image, the fax machine shown in FIG. 1 performs the above-described steps in reverse order. The general purpose processor block controls and performs the conversion of the hard copy image to a corresponding compressed bit map. The compressed bit map is then provided to the special purpose fax modem block which utilizes the bit map to modulate a carrier waveform which is transmitted over the analog channel to a destination fax machine.
A modem architecture similar to that of the Rockwell Module is also provided by the Yamaha YM7109 FAX modem LSI chip.
The fax machine architecture exemplified by the Rockwell Module and the Yamaha modem chip, that is, a special purpose fax modem block in combination with a separate general purpose processor block, suffers from a number of disadvantages. First, the system requires two separate processor functions: the special purpose DSP function of the modem block for recovering the compressed bit map and the general purpose processing and control functions of the general purpose processor block for performing the remaining tasks required to convert the compressed bit map to hard copy. Since there are periods of time when no facsimile transmissions are being received, the system's full processing capability is greatly underutilized. Furthermore, the DSP functions of the modem block are dedicated to a particular application, in this case, facsimile reception/transmission. That is, as stated above, the DSP algorithm utilized to recover the incoming data is fixed; aside from the ability to modify the coefficients of the "fax" algorithm, there is no flexibility in the modem algorithm to allow it to perform tasks other than facsimile data recovery. This results in a high-cost, application-specific system architecture with redundant processing capabilities.
A variation in the Rockwell and Yamaha modem architectures is exemplified by the OKI KV96-X6D modem chip set. While the architecture of the OKI modem chip set maintains the separate modem and general purpose processor functions of the Rockwell and Yamaha modems described above, its analog front end and DSP functions are also separated. Since the DSP function is programmable, some flexibility in the type of signal that may be processed is permitted. However, once programmed, the DSP function of the OKI modem still relies on a fixed DSP algorithm. Thus, the OKI architecture has the same basic limitations and inefficiencies as the Rockwell and Yamaha devices.
The Texas Instruments TMS320C25 Digital Signal Processor provides a "general purpose" DSP capability in that it can accommodate a number of DSP algorithmic sequences. However, it relies on dedicated memory for storage of its DSP operations and data. Thus, it must incorporate its own segregated control capability aside from that provided by the general purpose processor with which it is associated.
NEC IC Microsystems Ltd. provides a modem DSP chip that includes a DSP core that is integrated with a general purpose processor block. However, the DSP core of the NEC device is dedicated to a particular algorithm and relies on its own control functions and an internal memory separate from that of the general purpose processor function for storage and retrieval of its operands. Furthermore, the general purpose processor function is fully embedded, making it unavailable for tasks other than those related to the dedicated DSP function.
It would, therefore, be desirable to have available a dual processor platform that can execute a variety of DSP algorithms while maintaining full general purpose processor capability.